Local and systemic changes in the acute phase proteins, haptoglobin and serum amyloid A (SAA), were studied in six dairy cows during the acute and chronic phases of experimentally induced Staphylococcus aureus mastitis. Haptoglobin and SAA were measured in serum, and in milk from infected and healthy control udder quarters within each cow. Concentrations of haptoglobin and SAA increased rapidly in both serum and milk during the acute phase of mastitis and followed a similar pattern. Significantly raised milk concentrations of SAA were also found during chronic subclinical mastitis. Serum concentrations of SAA also tended to be higher during the chronic phase than pre-infection. Increases in milk haptoglobin and SAA were specific for the infected udder quarters. In conclusion, measurement of SAA in milk samples could be a useful tool in diagnosing mastitis.